In the prior art, when, for example, manufacturing a so-called MEMS device including miniaturized mechanical components and electronic components, dry etching is employed to etch silicon layers used in the MEMS device. Patent document 1 describes the employment of reactive dry etching to form a pattern of recesses, through holes, and the like in a silicon layer of the MEMS device. A dry etching method of the prior art will now be described with reference to FIG. 4 based on patent document 1.
As shown in FIG. 4(a), in the dry etching, plasma using sulfur hexafluoride (SF6) gas, that is, an etchant 54 containing fluorine radicals (F*) and various types of positive ions is generated in a vacuum container, which accommodates a substrate S that is subject to processing. The substrate S includes a silicon layer 52, which forms an MEMS device. The silicon layer 52 is, for example, stacked on a silicon oxide layer 51, which is an etching stopper layer. An etching mask 53 is formed on a surface 52s of the silicon layer 52. The etching mask 53 is patterned to expose an etched region 52a of the silicon layer 52. Then, as shown in FIG. 4(b), the positive ions drawn into the substrate S by bias voltage applied to the substrate S and the fluorine radicals contacting the surface of the substrate S advances the etching reaction in the etched region 52a and forms a recess 55 in the etched region 52a. 
The positive ions drawn into the substrate S advance etching in a thicknesswise direction of the silicon layer 52. However, the radicals that are not directive advances etching not only in the thicknesswise direction of the silicon layer 52 but also in a direction intersecting the thicknesswise direction. In the MEMS device, the thickness of the silicon layer 52 is tens of micrometers to hundreds of micrometers. Thus, when continuously performing isotropic etching with such radicals over the entire thickness of the silicon layer 52, the recess 55 formed in the silicon layer 52 may greatly extend not only in the thicknesswise direction of the silicon layer 52 but also in the direction intersecting the thicknesswise direction. With regard to such a problem, the dry etching method described in patent document 1 is performed in the following manner.
In this method, the etching reaction is temporarily stopped after the recess 55 is partially formed one in the thicknesswise direction of the silicon layer 52, as shown in FIG. 4(b). Then, as shown in FIG. 4(c), hydrocarbon trifuloride (CHF3) gas 56 is sent into the vacuum container to form a protective film 57 of polytetrafluoroethylene ((C2F2)n) over the entire surface of the substrate S including an inner surface of the recess 55. Subsequently, as shown in FIG. 4(d), sulfur hexafluoride gas is sent again into the vacuum container and plasmatized to resume etching in the thicknesswise direction of the silicon layer 52.
In this state, only the radicals mainly contact the protective film 57 formed on the side surface of the recess 55. In contrast, not only the radicals but also the positive ions contact the protective film 57 formed on the bottom surface of the recess 55. As a result, the removal of the protective film 57 with the etchant 54 is faster at the bottom surface of the recess 55 than at the side surface of the recess 55. Thus, the side surface of the recess 55 is protected by the protective film 57, whereas the bottom surface of the recess 55 is further etched in the thicknesswise direction of the silicon layer 52. Then, the etching step (FIG. 4(d)) using sulfur hexafluoride gas and the protective film forming step (FIG. 4(c)) using the hydrocarbon trifluoride gas 56 are alternately repeated until the bottom surface of the recess 55 reaches the surface of the silicon oxide layer 51. This forms a through hole H extending through the silicon layer 52, which has a thickness of tens of micrometers to hundreds of micrometers in the thicknesswise direction.